166 FOUNDATIONS OF BIOLOGY 



to flow in one direction only; that is, from auricle to ventricle. 

 An enlargement, the SINUS VENOSUS, connects the veins 

 (VENOUS SYSTEM) with the auricle, and there is frequently 

 another, called the CONUS ARTERIOSUS, in a similar position at 

 the arterial end. The heart is thus essentially a linear series 

 of chambers. The sinus venosus and auricle function mainly 

 as reservoirs to fill rapidly the especially muscular ventricle. 

 The latter, acting both as a suction and force pump, passes 

 the blood on to the conus arteriosus and from there to the 

 ARTERIAL SYSTEM as a whole. For our purposes, however, we 

 may consider the heart in the lowest Vertebrates (Fishes) as 

 composed of the two chambers, auricle and ventricle. (Fig. 91.) 



The arterial system is the distributing system of vessels 

 which carries the blood to all regions of the body. Soon after 

 its origin at the heart the circuit in the aquatic forms is tempo- 

 rarily interrupted to allow the blood to pass through the GILLS 

 and exchange carbon dioxide for a supply of oxygen. < To 

 facilitate this gaseous interchange, the arteries (AFFERENT 

 BRANCHIAL) as they enter the gill membrane break up into 

 smaller and smaller vessels which finally are of microscopic 

 calibre and consist of but a single layer of cells. These capil- 

 laries, in turn, merge into larger vessels (EFFERENT BRAN- 

 CHIAL ARTERIES) which finally lead into the chief artery of 

 the body, the DORSAL AORTA^ This extends along the median 

 dorsal line of the body, just below the vertebral column, and 

 sends branches to the various organs. 



The branches of the dorsal aorta, on reaching the location 

 which they supply with arterial blood, break up into capil- 

 laries similar to those in the gills, and pass to the tissues 

 the blood carrying food and oxygen. The blood receives in 

 return various waste products of metabolism, including car- 

 bon dioxide and, in certain cases, absorbed food materials 

 from the intestine, and special secretions chiefly from ductless 



